The invention relates to the field of phacoemulsification probe driving apparatus, and more particularly, to the field of tuned probes for phacoemulsification.
It has long been known that, in delivery of electric power to inductive loads or capacitive loads, maximum efficiency and maximum delivery of said power occurs when the phase angle between the voltage across the load and the current through the load is substantially zero. The phase angle of a system is related to the power factor. Those skilled in the art appreciate that the impedance of any network which includes inductive or capacitive elements in addition to resistive elements is the vector sum of the real component, i.e., the resistive elements, and the imaginary component caused by the presence of the inductive and capacitive elements. If the reactive component is zero, then the impedance of a system is purely resistive, and the resultant vector is coincident with the real axis. In such a circumstance, the phase angle is zero. Power factor is a measure of the relative magnitudes of the reactive and real components in a load impedance. It is related to the relative magnitude of these two vector components.
Power factor is also a measure of the efficiency of a system in delivering power to a load. Since only resistive components can actually dissipate power, the presence of an inductive or capacitive reactance component in a load impedance will decrease the efficiency of power delivery of the system, since it causes increased power dissipation in the source resistance of the power supply. The reason for this is well understood by those skilled in the art and will not be detailed here. As a consequence of the foregoing reality, it has long been known by utility companies and other practitioners of the power delivery art that to maximize the efficiency of power delivery to a load, it is useful to tune out the reactive component of the load impedance by placing it in series or parallel with an equal and opposite sign reactive component in a tuning circuit so that the resultant load impedance is purely resistive. In such a circumstance the source impedance is said to be the matched conjugate of the load impedance, and the power delivered to the load is maximized.
Power delivered to a load is given by the following expression: EQU Power=VI cos.THETA. (1)
where V is the voltage drop across the load impedance, and I is the series current flowing through the load impedance, and cosine theta is the power factor of the circuit. The power factor is said to be "leading" if the current leads the voltage, and "lagging" if the current lags the voltage.
Ultrasonic probes have traditionally been used for phacoemulsification for rupturing of cataracts in the eye coupled with aspiration of the pieces of tissue disrupted by the probe. There have been developed two classes of probes, one of which is excited by piezoelectric crystals. Such piezoelectric probes traditionally have been rods of metal, such as titanium, having piezoelectric crystals affixed therein to act as excitation sources to cause the rods to vibrate. The piezoelectric crystals are driven with electrical alternating current driving signals having high frequencies, such as 40.000 Hz. The length of the probe is such that it is a multiple of one-half the wavelength of the driving signal. Vibration of the piezoelectric crystal under the influence of the driving signal causes the rod to vibrate at its mechanical resonant frequency.
The piezoelectric crystals which are used as excitation sources in such probes, when coupled with the mass of the probe rod, can be modeled as an equivalent electrical circuit having inductive, capacitive, and resistive components. There is a capacitive component representing the elasticity of the metal of the rod and and inductive component representing the mass of the probe. There is also a resistive component representing resistance to motion of the tip of the rod as it hits loads such as tissue or fluids in the eye which tend to dampen the vibration of the tip of the probe. The piezoelectric crystal itself contributes a resistive component which is related to the amount of leakage of current between the terminals of the crystal. The crystal also has a capacitive component which represents the intrinsic electrical characteristics of piezoelectric crystals, i.e., the thickness and the dielectric constant and the area.
As the temperature changes, and as load on the probe changes, the various resistive and reactive components in the equivalent circuit of the probe change values. These changes in the component values change the mechanical resonant frequency of the probe. Unless the driving frequency is changed to correspond with the changed resonant frequencies, maximum power-transfer efficiency will not be achieved.
Further those skilled in the art understand that maximum power transfer between a source and a load occurs when the impedances of the source and the load are matched so that the load appears to be purely resistive. Therefore, in the case of an ultrasonic probe if the probe load impedance at the resonance frequency has a capacitive reactive componente, the source impedance should have an inductive reactive component of equal magnitude to maximize power transfer between the source and the load. Because of the changing magnitudes of the resistive and reactive components of the combined mechanical and electrical system of a phacoemulsification probe, as the power level changes and as the temperature and load conditions of the probe change, it is difficult, if not impossible with a fixed inductor to match the source impedance to the load impedance to cancel out the probe's reactive component over a broad range of power levels and frequency variations. An advantage of such a matched, tuned system is that low voltage components may be used since the impedance seen by the source voltage generator is minimized (looking into a two-port network including the tuning inductor).
Accordingly, there has arisen a need for a phacoemulsification probe driver which can be tuned such that the reactive component of the load is cancelled as conditions such as power level, temperature, and loading change. Further, there has arisen a need for a probe driver circuit which can alter the driving frequency to match the changed mechanical resonant frequency as power level, temperature, and loading conditions change. Further, a need has arisen for a phacoemulsification probe driver with proportional power control such that the user may set a desired power level and that level of power will be transmitted to the probe.